Find the incorrect statement about plasmids

To identify the incorrect statement about plasmids, let us analyze their key biological characteristics step-by-step:

1. Nature of Plasmids: Plasmids are small, extrachromosomal DNA molecules found most commonly in bacteria, though they can also be found in archaea and multicellular organisms.

2. Structure: The standard structure of a plasmid is double-stranded and circular. Thus, the statement "they are circular" is correct.

3. Replication: Plasmids possess their own origin of replication ($\mathrm{ori}$), allowing them to replicate autonomously and independently of the chromosomal DNA. Thus, the statement "they replicate independently" is correct.

4. Transferability: Many plasmids contain genes that facilitate conjugation, enabling them to be transferred from one bacterium to another. Thus, the statement "they are transferrable" is correct.

5. Strandedness: Naturally occurring plasmids are double-stranded DNA molecules, not single-stranded. Therefore, the statement "they are single stranded" is false.

Hence, the incorrect statement about plasmids is "they are single stranded".